Ruthenium mu-oxo dimers with chloro-substituted 2,2’-bipyridines [L(2)(H2O)(RuORuIII)-O-III(OH2)L(2)](4+) (L = 4,4’-dichloro- or 2 5,5’-dichloro-2,2’-bipyridine) were synthesized. Electrochemical studies on the complexes showed that the Ru-III-O-Ru-III dimer undergoes a reversible one-electron oxidation to Ru-III-O-RU(IV) followed by a reversible multi-electron oxidation assignable as the oxidation of Ru-III-O-Ru-IV to Ru-IV-O-Ru-V. The Ru-III-O-Ru-IV and Ru-IV-O-R(V) species were characterized by thin-layer spectroelectrochemistry. The Ru-IV-O-Ru-V dimer is an active catalyst for the chemical or electrochemical oxidation of water to dioxygen. Upon reduction, the Ru-III-O-Ru-III dimer undergoes reductive cleavage to give [Ru(II)L(2)(H2O)(2)](2+). The chloro-substituted ruthenium dimers are more stable towards reductive cleavage in mildly acidic media than the unsubstituted [(bpy)(2)(H2O)(RuORuIII)-O-III(OH2)(bpy)(2)](4+) (bpy = 2,2’-bipyridine).